The purpose of this project is to determine the influence of virus load and CD4+ T-cell counts, biologic phenotype and the presence of symptoms of HIV disease on responses to new therapeutic regimens in HIV infected individuals who have been treated with either ZVD or ddI monotherapy, and on the development of polymerase gene mutations associated with antiretroviral drug resistance. Additionally, the investigators hope to determine the antiviral effects of each of the defined treatment groups.